This invention relates generally to the field of computer graphics, and relates more particularly to an apparatus for scan conversion, the conversion of information describing a picture from a compressed form easily produced by a computer to an expanded form easily displayed by a raster scan device.
While applicable to other raster scan devices, this invention is primarily directed towards cathode ray tube display equipment. The cathode ray tube displays used for computer graphics may be divided into two basic catagories, calligraphic displays and raster scan displays.
Calligraphic displays are devices in which the electron beam scans an arbitrary pattern. Typically, these devices consist of a display file memory containing pieces of information describing a picture passed to a display processor, which calculates the positioning signals for the cathode ray tube display. Pictures are formed by the path travelled by the electron beam.
Raster scan displays are devices in which the electron beam scans a predefined pattern of scan lines. Typically, these devices consist of a display file memory containing pieces of information describing a picture passed to a display processor, which calculates pieces of information describing positions on the display raster collected in a frame buffer memory. The frame buffer memory is a random access storage device containing separate pieces of information corresponding to the attributes, such as intensity, color, and saturation, of positions defined on the display raster. These pieces of information are accessed in synchrony with the scan of the display to deliver the information describing the attributes of the screen as it is required. Pictures are formed by the variation of the attributes of the screen.
A general discussion of computer graphics displays may be obtained by referring to the article entitled "Computer Displays" by I. E. Sutherland, appearing in Scientific American, June 1970, page 57.
While possessing the ability to display shaded and colored pictures on low cost displays, raster scan displays have not generally been favored for computer graphics due to the high cost of sufficient storage to serve as a frame buffer memory and the extreme latency involved in transferring changes made in the display file memory to the frame buffer memory. These drawbacks to raster scan displays are purely the consequence of the use of the frame buffer memory, rather than an inherent quality of raster scan displays. To operate a raster scan display without the use of a frame buffer memory requires finding a means to perform the scan conversion at the rate of the scan of the display.
Detailed examples of computer graphics systems employing raster scan displays without the use of a frame buffer memory are given in the following U.S. Pat. Nos.: 3,812,491--Barraclough, et al (1974), 3,893,075--Orban, et al (1975), and 3,996,585--Hogan, et al (1976). Of a similar construction to conventional raster scan computer graphics systems using a frame buffer memory, these devices consist of a display file memory passing pieces of information describing a picture to a display processor, which calculates pieces of information describing positions on the scan line collected in a scan line buffer memory. The scan line buffer memory is a random access storage device containing separate pieces of information corresponding to the attributes of positions defined on the scan line. Since the display processor must now examine those pieces of information in the display file memory contributing to the appearance of the scan line at least once to determine the appearance of that scan line, rather than once for the display of the screen, it must operate at a very high rate. In the prior art, this requirement was accomodated by incorporating a display processor designed to operate on consecutive scan lines. This restriction permitted the retention of pieces of information describing the previous scan line. By merely calculating the incremental changes to this information, the display processor is able to operate at the rate required. Unfortunately, modification of the information in the display file memory must only occur when no pieces of information are being retained, for the display to accurately reflect the picture described by the pieces of information in the display file memory. Ordinarily, this situation will only occur during a short period between the successive displays of the screen. This imposes a significant limitation on the time available for modification of the pieces of information in the display file memory.
Although other examples of computer graphics systems employing raster scan displays without the use of a frame buffer memory exist, systems of the form of a display file memory passing pieces of information describing a picture to a display processor, which calculates pieces of information describing positions on the scan line collected in a scan line buffer memory enjoy considerable advantages in terms of cost effectiveness and high performance over competitive systems.
Therefore, it is a principle object of the present invention to provide an apparatus for scan conversion that is cost effective and of high performance, yet allows greater accessibility for the acceptance of picture information.
It is another object of the present invention to eliminate the cost of a supplemental memory to retain pieces of information describing the previous scan line.